Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. When transitioning between a VDE mode (where one or more cylinders are deactivated) and a non-VDE mode (where all the cylinders are active), the control system may adjust one or more engine operating parameters to reduce disturbances (e.g., torque disturbances) and attenuate the disturbance during the transition.
One example approach for engine control during a VDE transition is shown by Pallett et al in U.S. Pat. No. 7,225,782. Therein, the VDE engine is coupled in a hybrid electric vehicle having an electric motor. When enabling or disabling a cylinder, torque from the motor is varied to compensate for transient changes in engine output torque caused by the enabling or disabling of the cylinder.
However the inventors herein have identified potential issues with such an approach. As one example, combustion stability may be degraded during the transition. Specifically, when transitioning from the VDE mode (or partial cylinder mode) to the non-VDE mode (or full cylinder mode), cylinder load decreases based on the decrease in aircharge. The lighter cylinder loads generally have less stable combustion and the interaction with the transient fuel compensation, and other cylinder conditions that are different than the operating cylinders due to cooling during deactivation may contribute to less stable combustion during reactivation. If the engine is equipped for exhaust gas recirculation, EGR control used during the transition, it may exacerbate the combustion issues. In particular, the EGR may continue to interfere with the lighter cylinder load until the EGR delivered to the cylinders has been sufficiently bled down to reduce combustion issues. In some embodiments, charge motion control valves (CMCVs) may be used to adjust the in cylinder motion of the air fuel mixture delivered to the cylinder during the transition. High cylinder motion results in better mixing, and more stable combustion. However, due to the slower response time of the CMCV (e.g., the CMCV not shutting quickly enough when transitioning to the lower cylinder load), combustion stability may be compromised. The poor combustion conditions can also lead to slow burns or even misfires. Overall, combustion stability and engine performance may be degraded.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: selectively deactivating one or more engine cylinders responsive to operating conditions, and during reactivation of the cylinders, adjusting ignition energy of the reactivated cylinders for a number of combustion events. Specifically, the ignition energy may be temporarily increased during the reactivation. In this way, combustion stability is improved and torque disturbances are reduced during a transition out of a VDE mode of operation.
In one example, a variable displacement engine may be configured with selectively deactivatable fuel injectors. In response to selected deactivation conditions, such as reduced engine load or torque demand, one or more cylinders may be deactivated and the engine may be operated in a VDE mode. For example, the engine may be operated with half the cylinders deactivated. During the deactivation, an ignition energy of the active cylinders may be adjusted based on the engine speed-load conditions. Then, in response to selected reactivation conditions, such as increased engine load or torque demand, the deactivated cylinders may be reactivated and the engine may resume a non-VDE mode. During the reactivation, for a number of combustion events since the reactivation, the ignition energy of the cylinders may be increased. For example, the dwell time of the ignition coil may be increased. Additionally or optionally, a number of strikes of the ignition coil may be increased. After a number of combustion events, a nominal ignition energy may be resumed.
In some embodiments, such as where the engine was operating with EGR during the VDE mode of operation, the increasing of the ignition energy during the reactivation may be adjusted based on the EGR. Specifically, the ignition energy may be increased while the EGR is bled down. By using a higher ignition energy at the time of reactivation, a higher EGR rate can be used in VDE mode because the EGR can be bled down from this higher rate and the transition to the lower cylinder load can be advanced without degrading combustion. Alternatively, the transition to cylinder reactivation can be performed at a higher EGR level during the EGR bleed down period than would have been possible without the enhanced ignition output, reducing the delay time before transition to cylinder reactivation.
In this way, by selectively increasing the ignition energy during the reactivation of engine cylinders, combustion stability is improved during a transition from VDE mode to non-VDE mode of engine operation. The increased ignition energy also renders torque disturbances during the transition substantially imperceptible to the vehicle operator. By increasing the ignition energy temporarily, rather than using a high ignition output continuously during all engine operating conditions, component durability issues resulting from the high heat output are reduced. Further, the life of spark plug components, such as the ignition electrodes, is prolonged. Additionally, transition to cylinder reactivation can be performed during a higher EGR level during the EGR bleed down phase, improving the response time of reactivation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.